Ammonia gives a self-announcing leak: its smell is noticeable at 5 to 50 ppm, well below the 300 ppm the carrier calls toxic and far below the 2,500 ppm it says can be fatal within 30 minutes, so a leak is usually noticed long before it can kill. The matching drawback is that it corrodes copper and copper alloys, so an ammonia plant must be steel throughout, including any fitting someone swaps in later without knowing why.
Part of Refrigeration
How ammonia harms the body, why its smell is an unusually good alarm and where that alarm fails, and the ventilation, detection and response measures that follow.
The Smell Gives A Margin, And Three Things Erode It
The carrier’s figures are these: smell at 5 to 50 ppm, immediate irritation and a toxic level at 300 ppm, potentially fatal within 30 minutes at 2,500 ppm, and flammable at 15 to 28% in air. At the top of the smell range there is a factor of 300 ÷ 50 = 6 to the toxic figure, and at the bottom a factor of 300 ÷ 5 = 60. Two other refrigerants in this tree give no such warning: raw propane is odourless unless something is added, and carbon dioxide has no smell.
The margin shrinks in three ways.
- The nose adapts. A person who stays in a faintly smelling room notices it less over time (recalled behaviour, flagged). A smell noticed on entering is a warning; a smell that has faded is not a clearance.
- The people who need the alarm most may not get it. Someone asleep, intoxicated, or with a cold does not notice.
- A big release does not build slowly. A fracture can release far more than the smell range, in seconds, and the first sign may already be a sore throat and streaming eyes.
The harm mechanism is that ammonia dissolves in the moist surfaces of eyes, nose, throat and lungs and forms an alkaline solution there (recalled chemistry, flagged), so it burns tissue as well as irritating it. The danger rises with concentration and time; the carrier’s first-aid advice is to move to fresh air, flush eyes and skin with large amounts of water for at least 15 minutes, and seek medical attention for any breathing difficulty. This page gives no further treatment.
What You Need
- Ventilation at high and low level. Vapour at room temperature is lighter than air (molar mass 17 against 29), so an opening at the top lets it escape; a cold release from a liquid can hang low at first, so an opening at the bottom helps too (recalled behaviour, flagged).
- Leak detection without flame: soapy water on joints, a wet pH indicator strip near joints, and the nose.
- Water supply and an eyewash within a few steps of the plant.
- Two exits and doors that open outward, with the plant outdoors or in a separate building.
- Steel everywhere ammonia touches, and a label saying so.
Method: Respond To A Leak
Step 1: Cut the heat or the power. For an absorption machine, remove the fire or bath. The pressure that drives the leak falls only when the heat stops.
Step 2: Get yourself and others out, upwind. Do not go back in for anyone; get them out from the outside if you can, and raise help.
Step 3: Ventilate from outside. Open doors and vents at both levels, from the outside, before anyone goes in.
Step 4: Find the leak with soap and an indicator strip, from a distance and upwind, once the air allows.
Step 5: Do not use a flame at any point. The ammonia is flammable in a band, and older practice of burning sulfur to see white fume near the joint puts a flame in the vapour (recalled practice, flagged).
Step 6: Record what happened. Date, cause, steps and repairs go in the machine’s log.
Worked Example: How Fast A Fault Raises The Concentration
Take the closed room of 30 m³ and a slow leak. An assumed release of 1 g of ammonia per minute adds:
- Moles per minute: 1 ÷ 17 = 0.0588 mol.
- Gas volume per minute at about 24 litres per mole: 0.0588 × 24 = 1.41 litres, which is 0.00141 m³.
- Concentration added per minute: 0.00141 ÷ 30 = 4.7 × 10⁻⁵, which is 47 ppm.
At 47 ppm a minute, the room passes the carrier’s 300 ppm level in about 300 ÷ 47 = 6.4 minutes, and its 2,500 ppm level in about 2,500 ÷ 47 = 53 minutes, with no ventilation at all. The leak rate is an assumption chosen for the arithmetic; the point is that the smell comes first, at the carrier’s 5 to 50 ppm, in the first minute, leaving most of an hour to act. A burst is not a slow leak and gives no such time.
Common Mistakes
- A brass or copper fitting “as a temporary measure”. It becomes permanent and fails first.
- Treating a faded smell as an all-clear.
- Sealing the plant room to keep warm. The ventilation is the safety.
- Mixing ammonia with chlorine bleach or other chlorine products, which release toxic gases (recalled chemistry, flagged). Keep them in separate places.
- Flushing ammonia into a stream or pond. It poisons fish at low concentrations; see pH and Ammonia.
Safety
The warning smell is not a licence to stay
Ammonia in a closed room can pass the carrier’s fatal level in the time it takes to find the leak. If you smell it strongly, leave, upwind, and ventilate before anyone returns; never enter a space that smells strongly without a second person outside and a way out. A release in a cold room, a pit or a shed with no through-ventilation is the case that kills. The machine must also have a relief device and a heat source you can remove; see Pressure Vessels.
What’s Next
- Learn next: Pressure Vessels — the other half of plant safety
- Combine with: Troubleshooting — diagnosing the machine that produced the leak